An Overview of Synchrotron Techniques for Studying Environmental Processes Paul Northrup Brookhaven National Laboratory Environmental Sciences Department Environmental Research & Technology Division “Nothing is so difficult but that it may be found out by seeking.” -Terence (ca. 150 BC) NCSS July 18, 2006
25
Embed
An Overview of Synchrotron Techniques for Studying Environmental Processes
An Overview of Synchrotron Techniques for Studying Environmental Processes. Paul Northrup Brookhaven National Laboratory Environmental Sciences Department Environmental Research & Technology Division. “Nothing is so difficult but that it may be found out by seeking.” - PowerPoint PPT Presentation
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
An Overview of Synchrotron Techniques for Studying Environmental Processes
Paul Northrup Brookhaven National Laboratory
Environmental Sciences Department
Environmental Research & Technology Division
“Nothing is so difficult but that it may be found out
by seeking.” -Terence (ca. 150 BC)
NCSS July 18, 2006
The National Synchrotron Light Source - User facility- 300mA, 2.8 GeV- IR >>>> 100KeV
Techniques:
• X-ray absorption:
– absorption spectroscopy (XAS)
– fluorescence (XRF)
– microscopy
• X-ray scattering:– diffraction
• IR spectroscopy/microscopy
Applications of Synchrotron Techniques
• Complex environmental systems and how contaminants interact
• Imaging/mapping elemental distributions• A probe of chemical and structural state:
– Oxidation state and chemical bonding– Local and long-range structures– Reactivity, mobility, bioavailability (toxicity)– Biological & geochemical processes
• Element-specific and Non-destructive• Trace or major components, processes
Xray Absorption Spectroscopy: Each edge of each element has a characteristic binding energy
Sulfur K edge
M
L
K
Absorption occurs when the energy of the incident photon is sufficient to eject the electron.
XAS measurement
• Direct: transmission through sample.
Absorption = ln(Io/It)
XAS measurement
• Indirect: X-ray fluorescence produced as electron “hole” is filled.• Characteristic energy for each element. • Proportional to absorption.
Three components of XAS:
• Edge step• Electron transitions• Extended
oscillations• Each carries different information.
Absorption edge step
Absorption (step height) is proportional to concentration.
Eo indicates oxidation state, by small shifts.
<<<<<reduced oxidized>>>>>
Electron transitions• Promotion of electron to available (unfilled) level of absorbing atom -- or neighbor. • Peak energy differs from edge energy. • Sensitive to electronic configuration and bonding. • Rules: Allowed:
S K edge• 2 edge steps (oxidation states) • 1s to 3p electron transition: 1: sulfide/thiol (R-S-R/R-SH), 2: thiophene, 3: sulfoxide (R-(SO)-R), 4: sulfite/sulfone
(R-OSO2-/R-(SO2)-R),
5: sulfonate (R-SO3-),
6: sulfate (R-OSO3-)
1 2 3 4 5 6
Organic S species
• Sulfur in sediments • Sulfate (bio)reduction
• Sulfur in plant roots • Physiological response to toxin (Zn)
EXAFS• Extended oscillations due to backscatter of electron from neighboring atoms • Interference pattern: • Distance • What element (size)• Coordination number • U incorporation into a
P K edge: • P interacts with U• Oxidation state • Organic/inorganic species• 1s to 3p transition
Phosphate in solution• Structural response to pH • Degree of protonation induces shift in peak: (PO4)3- vs. H3PO4
• Transformation of organic phosphate ester to free phosphate• Action of microbial phosphatase• Uranium
Identifying phosphates:
• (1) Presence of Ca bound to phosphate oxygen creates new transition• (2) Uranium phosphate• (3) Fe phosphate
Phase identification:
• Sometime quick “fingerprinting” is possible• Calcite vs aragonite -- both CaCO3
Microbeam XRF, XAS• Map concentration of major and trace elements• Analyze species and structure at isolated points• U association with Fe oxides• U incorporation into calcite• Pu with Mn oxides• U reduction at Fe(II)/Fe(III) surfaces• ID minor components, precipitates• Interactions of contaminants with plants, microbes
Fe
U
“Soft” X-rays:
• STXM• C, N, O edges, very low energy• Spectral analysis to image distribution of different organic compounds and oxides• Resolution ~30 nm• Image distribution of contaminants within/around single cells:
• bioreduction, metabolism, toxicity
IR microscopy/spectroscopy:
• Vibrations rather than electronic effects• Organic functional groups, ID and distribution• Correlations with metal distribution
X-ray Diffraction:
• Planes of atoms in a crystalline solid diffract X-rays• Diffraction angle depends on spacing between layers• Crystal structures have unique diffraction patterns• Identify crystalline phases: minerals, precipitates•Two types: • Powder diffraction: ID major and minor components in bulk samples• Microdiffraction: ID individual grains
Summary:
• Several synchrotron tools are useful to study molecular-scale and bulk chemical and processes in the environment• Most questions are best addressed using a combination of techniques